Spray dryers generally contain a separator, more particularly a cyclone, situated outside the drying container and a fan cooperating therewith. In the separator, the sprayed material being dried is separated from the used gas. The excess pressure or reduced pressure required for operation and the corresponding gas flow are regulated by means of the fan. In all installations of this type, the dust initially leaves the drying container or tower and, thereafter, has to be separately transported or returned to the tower.
In externally arranged separators, particularly cyclones or filters, the cross-section of the pipe for the dust-laden untreated air between the drying container and the separator is gradually reduced by caking dust, necessitating cleaning at correspondingly regular intervals. In addition, separate means for collecting and discharging the dust separated off, in the form of discharge funnels and locks, have to be associated with the external separators. This also increases cleaning and maintenance costs, in addition to which personnel are endangered by the emission of dust during the cleaning and maintenance work.
The externally accumulating dust may also be returned by injection into the spraying zone or, after mixing with water, by introduction into the slurry (wet drying stock). In the first instance, dust accumulates in the spraying zone to such an extent that the temperature and moisture content in that zone fall, so that the dust does not agglomerate as well. The alternative moistening of the dust and its introduction into the slurry necessitates a higher consumption of energy for the additional drying step. Additionally, attempts could be made to add the externally accumulating dust to the dried material. However, this would only increase the percentage of fines in the product and produce an uneven distribution thereof (for example in the form of strands), with the result that the measuring techniques used for determining weight per liter and product moisture (i.e. the crucial quality criteria) would give inaccurate or incorrect results.
German Patent Application No. 17 79 260 describes a spray dryer in which the separator is arranged inside the container or tower of the dryer. In this case, the separator, in the form of a cyclone, is arranged in the middle of the tower, i.e. in the drying zone. Since the volume available for use as a drying chamber is reduced in this way, the installation of a cyclone in the drying zone presupposes corresponding enlargement of the drying tower, besides which a cyclone arranged in the middle of the tower is inaccessible, particularly for cleaning work. Even though, in the spray dryer known from German Application No. 17 79 260 the dust is separated off inside the drying tower, agglomeration is still not possible at this stage. Instead, if it is to be returned, the dust has to be delivered back, for example to the spraying zone, by external means in this instance. Accordingly, the problems associated with spray dryers with respect to dust separators arranged outside the drying tower remain, in addition to which the percentage of fines in the dried product is undesirably increased by the mechanical load applied in the cyclone.